1. Field of the Invention
The present invention relates to a permanent magnet motor comprising a plurality of permanent magnets provided in a rotor core and a washing machine provided with the permanent magnet motor.
2. Description of the Related Art
Magnetic flux is generated by permanent magnets in a permanent magnet motor of the above-described type. The magnetic flux is interlinked with stator windings. It has been desired that an amount of magnetic flux interlinked with stator windings (an amount of interlinkage magnetic flux) should properly be adjusted according to load driven by the permanent magnet motor. However, permanent magnets provided in permanent magnet motors are generally composed of a single type of permanent magnet. Accordingly, an amount of magnetic flux of the permanent magnets is usually constant. For example, when a permanent magnet motor comprises permanent magnets with a large coercive force, voltage induced by the permanent magnets during high-speed rotation is rendered excessively high, resulting in possible breakdown of electronic components. On the other hand, when a permanent magnet motor comprises permanent magnets with a small coercive force, output power during low speed rotation is reduced.
For example, Japanese patent application publication, JP-A-2006-280195 discloses a permanent magnet motor comprising two types of permanent magnets with different coercive forces provided in a rotor core. In the disclosed permanent magnet motor, the permanent magnets with a smaller coercive force are magnetized or demagnetized by an external magnetic field due to armature reaction (a magnetic field generated by electric current flowing into stator winding), whereby an amount of magnetic flux of the permanent magnets is adjusted.
However, both permanent magnets with large and small coercive forces are disposed in each part constituting one magnetic pole inside a rotor core in the permanent magnet motor disclosed by the above-referenced publication. More specifically, the permanent magnet motor of the above-referenced publication is constructed so that one magnetic pole is formed by a plurality of types of permanent magnets. As a result, the number of permanent magnets is increased and a cubic volume of each magnet needs to be rendered smaller, whereupon the structure of the motor is complicated.